This invention relates to an improved process for converting coal or similar solid carbonaceous materials. More particularly, this invention relates to an improved process for liquefying coal and similar carbonaceous substances.
As is well known, coal has long been used as a fuel in many areas. For several reasons, such as handling problems, waste disposal problems, polution problems and the like, coal has not been a particularly desirable fuel from the ultimate consumers point of view. Moreover, coal cannot be used directly, especially in areas where a liquid fuel is required, such as in the operation of automobiles, trucks and airplanes. As a result, oil and gas have enjoyed a dominant position, from the standpoint of fuel sources throughout the world.
As is also well known, proven petroleum and gas reserves are shrinking throughout the world and the need for alternate sources of energy is becoming more and more apparent. One such alternate source is, of course, coal since coal is an abundant fossil fuel in many countries throughout the world. Before coal will be widely accepted as a fuel, however, it is believed necessary to convert the same to a form which will not suffer from the several disadvantages alluded to previously and which will permit the use of those areas where a liquid fuel is normally required.
To this end, several processes wherein coal is liquefied have been proposed heretofore. Of these several liquefaction processes which have been heretofore proposed, those processes wherein coal is liquefied in the presence of a hydrogen donor solvent or diluent and molecular hydrogen appear to offer the greater advantages. In these processes, liquefaction is accomplished at elevated temperatures and pressures and a relatively broad range of gaseous and liquid hydrocarbon products is invariably produced.
For the most part, hydrogen donor solvents or diluents which are derived from the solid carbonaceous material subjected to liquefaction are employed in these processes. In these processes, then, the solvent or diluent is obtained by subjecting all or a portion of a particular distillate fraction of the liquefaction product to hydrogenation to increase the hydrogen donor species content thereof. It is, however, difficult to obtain a solvent containing more than about 50 to about 55 wt. % donatable hydrogen species. As a result, higher yields of of lower quality than would be obtained if a pure hydrogen donor solvent were employed. The higher gas yields, in turn, result in a higher consumption of molecular hydrogen without a corresponding improvement in liquid quality and the lower quality liquid product requires severe hydrotreating before an acceptable liquid is, generally, obtained. The need, therefore, for a liquefaction process wherein lower yields of gaseous products are realized and an improved quality liquid product is obtained without the inherent cost associated with the use of pure components capable of donating large percentages of hydrogen is believed readily apparent.